1. Field of Invention
This invention relates to a confocal optical scanner such as used, for example, in a confocal microscope; and more particularly, to an improved pinhole device used in such confocal optical scanner.
2. Description of the Prior Art
FIG. 1 depicts an exemplary confocal microscope using a confocal optical scanner; and FIG. 2 depicts an exemplary conventional pinhole disck used in a confocal optical scanner. In FIGS. 1 and 2, output light from a light source , not shown, is irradiated onto a pinhole disk 3, in which a plurality of pinholes 32 are defined in a spiral shape in plate 31, passing through a polarizer 1a and a beam splitter 2. Light passes through one or more of the pinholes 32, and then imping on sample 6, after passing through 1/4 wave plate 4 and objective lens 5. Light reflected from sample 6 returns through pinholes 32, in the same optical path, and an image of sample 6 can then be seen by the human eye via beam splitter 2, polarizer 1b and eyepiece 7. Pinhole disk 3 is rotated at a constant speed by motor 8. The focusing light directed at sample 6 is scanned by movement of pinholes 32 due to rotation of disk 3.
In the conventional confocal optical scanner, the plurality of pinholes 32 are formed in a spiral shape on disk 3 as seen in FIG. 2, and are disposed isometrically when seen in a radial row. This particular placement of holes is used because in fabricating a mask for the pattern of the entire pinhole disk, it is necessary to copy the pattern of a radial row one by one at equal angles. Accordingly, the pitch between the pinholes increases with increase in radius, that is, as the pinholes get further from the center of the disk. Thus, one disadvantage of the conventional pinhole arrangement is that it is darker at the outer periphery and lighter at the inner periphery in terms of the amount of transmitted light. Thus, non-uniformity in brightness occurs when optical scanning is at the inside and outside peripheries of the disk.
The conventional optical scanner has another disadvantage, namely, when the center of the spiral shaped pattern of pinholes 32 and the center of rotation of the disk 3 drift apart from each other, stripes are caused on an image plane when one image plane is constructed by one scan. As shown in FIG. 3(A), the stripes occur when a track, which is assumed to be a circle, rotates with a decentering value e and is observed through a window, an x coordinate of an observation point A traces a cycloid, which is approximately a sine wave, whose amplitude changes by a dimension 2e in 180.degree.. Consider the case when scanning of one image plane is measured by 90.degree.. When scanning is started at point B in FIGS. 3(A) and 3(B), the radius increases by e until scanning arrives at point A. Thus, whereas point B' of the pinhole pitch should be scanned in one image plane in 90.degree., in fact it passes through point A whose radius is greater by a decentering value e than point B'. Accordingly, the interval between points B' and A is scanned excessively, and the image plane is whitened. In contrast, when the interval between points B' and C is scanned, scanning is not enough, so that the image plane is blackened. In practice, the scanning is carried out in an arc and the stripe caused by decentering takes the shape shown in FIG. 3(C). To eliminate such stripes, the decentering value e must be reduced to nil. However, that is difficult to achieve. Another way to reduce the stripes is to construct the image plane by doubling the scanning. But in that case, even if the decentering can be kept large, the signal to noise ratio (S/N) increases. Thus, in conventional devices, it has been difficult to eliminate the stripes.
Another disadvantage to conventional devices is that the pinhole disk and motor cannot be separated or either one or the other exchanged with others.
A futher disadvantage of the conventional device is that the luminous utilization efficacy is limited. For example, using the pinhole disk of FIG. 2, if the total aperture of the pinholes 32 is 1% of the total disk area, then light irradiated on the disk will pass through the pinholes in proportion to the opening area, that is in this case 1%.
Moreover, along the same line, another disadvantage is that large amount of light is reflected from the disk surface not defining the pinholes. This becomes stray light, which is not desired. One way of stopping or reducing stray light is to use a beam stop 7a on eyepiece 7, a polarizer 1a, and a 1/4 wave plate 4. However, such means is not entirely successful in eliminating stray light, and is expensive.